This invention relates to loudspeakers, and, more particularly, to a method of assembling a loudspeaker in which deformation of the surround and tolerance stack-up which can create dynamic misalignment of the voice coil relative to the magnetic gap are substantially eliminated.
Loudspeakers generally comprise a frame, a motor structure, a diaphragm, a lower suspension or spider and an upper suspension or surround. In one common type of speaker, the motor structure includes a permanent magnet mounted between a top plate and a back plate, a pole piece centrally mounted on the back plate and a voice coil axially movable with respect to the pole piece. The voice coil includes a hollow, cylindrical-shaped former having an outer surface which receives a winding of wire.
One end of the diaphragm is connected to the surround, which, in turn, is mounted to the upper end of the frame. The lower suspension or spider is connected at one end to a seat formed in the frame at a point between its upper and lower ends. The free ends of the diaphragm and spider are mounted to the voice coil and support it within the magnetic gap between the pole piece and top plate of the motor structure, with the former of the voice coil concentrically disposed about the pole piece. In some speaker designs, a dust cap is mounted to the diaphragm in position to overlie the voice coil and pole piece to protect them from contaminants.
In the course of operation of a speaker of the type described above, electrical energy is supplied to the voice coil causing it to axially move relative to the pole piece and within the magnetic gap formed between the top plate and pole piece. The diaphragm, spider and surround move with the excursion of the voice coil and are intended to collectively maintain the voice coil concentric to the pole piece within the magnetic gap. Nevertheless, a pervasive problem associated with speaker operation involves misalignment of the voice coil which can create noise, distortion and/or speaker failure.
Problems with voice coil misalignment can be attributed to different factors, some involving techniques for manufacturing speaker components and others relating to the method of assembling the speaker components. Currently, it is typical for a manufacturer of loudspeakers to purchase the surround and diaphragm as a pre-assembled unit from an outside vendor. Often the same vendor manufactures both parts, and it is presumed that such vendor can more efficiently interconnect the diaphragm and surround at its facility than can the speaker manufacturer. Unfortunately, it is not uncommon for the combined diaphragm-surround assembly to be shipped to the speaker manufacturer in such a way that the surround becomes deformed.
In a conventional speaker assembly operation, the frame and motor structure are initially interconnected, and then the voice coil is secured within the magnetic gap of the motor by the lower suspension or spider. With the voice coil in position, the diaphragm-surround assembly is attached by first gluing the apex of the diaphragm along the outer surface of the former of the voice coil and then gluing the surround onto a flange at the upper end of the frame. The surround is allowed to xe2x80x9cfloatxe2x80x9d or move atop the flange as the glue dries.
One problem created by this assembly operation is due to tolerance xe2x80x9cstack-up,xe2x80x9d or accumulation, in the components assembled prior to affixing the surround. As noted above, the motor, frame, spider and voice coil are all interconnected before attaching the diaphragm-surround assembly, and the dimensional variations of these elements within design tolerances may result in a lack of concentricity between the voice coil, and frame at the completion of the final assembly step. Further, the deformation of the surround may also result in a lack of concentricity between the outside diameter of the surround and its inside diameter.
In theory, by allowing the surround to float or move along the flange at the upper end of the frame while its glue dries, problems with lack of concentricity due to tolerance stack-up and/or deformation of the surround are overcome. It has been found that this is not adequate in some applications, particularly high excursion speaker designs. The cross-sectional shape of the surround is a carefully engineered parameter in the overall speaker design, and substantially affects the excursion response of the loudspeaker particularly when the excursions are high. Once the surround has been deformed, it is unlikely to return to its original profile or shape, or cannot do so because of a lack of concentricity with the flange at the upper end of the frame. As a result, when a deformed surround is attached to the frame, one side or area of the surround is typically stretched or extended to some extent whereas another area is compressed. In response to movement of the voice coil in either direction, the area of the surround which was stretched during assembly reaches its limit of extension or compression before other areas of the surround. When one area of the surround extends to a greater or lesser extent than another area of the surround, it tends to tilt or twist with the movement of the voice coil, especially at high excursions. This, in turn, causes the diaphragm and voice coil to twist or tilt in the same fashion and can be characterized as dynamic misalignment. When the voice coil becomes misaligned with the magnetic gap of the motor structure by such tilting or twisting motion, the result is the creation of noise, distortion and potential speaker failure due to direct parts contact.
In addition to the misalignment of the voice coil noted above, current assembly techniques may create an initial offset in a vertical direction. The connection between the diaphragm and voice coil is typically quite tight to maintain concentricity, and to ensure that a good bond is created in order to avoid a failure at the diaphragm-voice coil interface. Conventionally, the apex of the diaphragm is placed along the exterior of the voice coil against the spider or some other vertical reference point on the former of the voice coil, irrespective of the position of the surround. But since the surround and diaphragm are typically interconnected and assembled as a unit, as noted above, it is not uncommon for the surround to be displaced in a vertically upward or downward direction due to tolerance stack-up within the already assembly components, e.g. the motor structure, frame, spider and voice coil. As a result of such vertical displacement, the surround can be initially extended or depressed and will not assume its desired vertical profile after the assembly operation is completed.
It is therefore among the objectives of this invention to provide a loudspeaker, and a method of assembling same, in which dynamic misalignment of the voice coil with the magnetic gap of the motor structure is substantially eliminated, and the incidence of noise and distortion is reduced.
These objectives are accomplished in a method of assembling a loudspeaker in which the motor structure and frame are interconnected, the voice coil is initially secured within the magnetic gap of the motor by the lower suspension and then the diaphragm and surround are installed by first affixing the surround to a flange at the upper end of the frame so that its outer diameter of the surround is concentric to its inner diameter, and then inserting the lower end of the diaphragm within a pool of adhesive carried by a well structure mounted to the exterior surface of the voice coil such that the diaphragm is allowed to move in both a lateral direction and a vertical direction within the well structure while the adhesive cures.
This invention is predicated on the concept of accommodating both deformation of the surround and tolerance stack-up within elements of the loudspeaker while permitting the surround to assume as close to its original, design shape and position within the speaker as possible. With respect to the shape of the surround, in the presently preferred embodiment a ring is formed in the flange at the upper end of the frame having a diameter which is equal to the original outside diameter of the surround. Because the inner diameter of the surround is constrained by its connection to the diaphragm, affixing the outer diameter of the surround to the frame in a position against the ring tends to cause the surround to assume its original shape with its inner and outer diameters concentric to one another.
Tolerance stack-up is accommodated by the manner in which the diaphragm is secured to the voice coil. In one presently preferred embodiment, a diaphragm structure is provided which comprises an upper diaphragm mounted to the surround and a lower diaphragm having the general cross-sectional shape of a xe2x80x9cWxe2x80x9d with an outer section connected to the surround and an inner section mounted to the upper diaphragm. A protrusion is formed at the juncture of the inner and outer sections of the lower diaphragm. A sleeve formed with an adhesive trough or well is attached to the outer surface of the former of the voice coil in position so that the protrusion of the lower diaphragm can be inserted within the interior of the well. The well has a bottom wall connected to a pair of opposed side walls which are spaced from one another in a lateral direction and extend generally vertically upwardly from the bottom wall. Upon assembly of the diaphragm and surround, the surround is affixed by adhesive against the ring on the flange at the upper end of the frame and then the protrusion of the lower diaphragm is inserted within a pool of adhesive located within the well of the sleeve carried by the voice coil. The protrusion of the lower diaphragm is allowed to move both in a lateral direction and in a vertical direction within the well before the adhesive cures to account for tolerance stack-up within the components of the speaker in each of the lateral and vertical directions.
In an alternative embodiment, a standard diaphragm is employed having an upper end connected to the surround and a lower end insertable within the well structure of a sleeve mounted to the voice coil. The same lateral and vertical adjustment of the position of the lower end of the diaphragm within the well is provided as described above, and therefore problems of lack of concentricity between the surround, diaphragm and voice coil created by tolerance stack-up are substantially eliminated.